The function, reliability and performance of semiconductor devices depend on the use of semiconductor materials and surfaces which are clean and uniform. Billions of dollars and countless man-hours have been spent developing, characterizing, and optimizing systems and processes for fabricating and processing semiconductor materials. A primary goal of this activity has been the fabrication of materials and surfaces that are extremely clean and that have predetermined and desired properties that are uniform, or vary uniformly, across the entire wafer. In order to characterize and optimize these processes and the resulting material, it is necessary to be able to inspect and measure surface or bulk cleanliness and uniformity. For real-time process control, it is necessary to be able to make many measurements across a surface at high speed, and to do so in a manner that does not damage or contaminate the semiconductor surface.
One method of inspecting and measuring surfaces utilizes a non-vibrating contact potential difference sensor. The non-vibrating contact potential difference sensor consists of a conductive probe that is positioned close to a surface, and is electrically connected to the surface. The probe and the surface form a capacitor. An electrical potential is formed between the probe tip and the surface due to the difference in work functions or surface potentials of the two materials. This electrical potential is called the contact potential difference, or surface potential difference, between the two surfaces. The probe tip is translated parallel to the surface, or the surface is translated beneath the probe. Changes in the work function or surface potential at different points on the surface result in changes in contact potential difference between the surface and the probe tip. These changes in electrical potential cause an electrical current to flow in or out of the sensor probe tip. This current is amplified, converted to a voltage, and sampled to form a continuous stream of data which represents changes in potential across the measured surface. The non-vibrating contact potential difference sensor can provide a continuous stream of data at rates greater than 100,000 samples per second. High data acquisition rates permit high-resolution images of whole semiconductor wafers to be acquired in only a few minutes.
The non-vibrating contact potential difference sensor produces a signal that is a combination of two characteristics of the measured surface—changes in work function and changes in surface height. The charge on the probe tip is determined as follows:Q=CV  (1)
Where Q is the charge on the probe tip, C is the capacitance between the probe tip and the measured surface, and V is the contact potential difference between the probe tip and the surface.
The current, i, into the probe tip is the derivative of the charge on the probe tip and is given by the following formula:
                    i        =                                            ⅆ              Q                                      ⅆ              t                                =                                    C              ⁢                                                ⅆ                  V                                                  ⅆ                  t                                                      +                          V              ⁢                                                ⅆ                  C                                                  ⅆ                  t                                                                                        (        2        )            
The current, i, is the sum of two terms: the dV/dt term and the dC/dt term. The dV/dt term represents changes in the voltage between the probe tip and the wafer surface, and the dC/dt term represents changes in the capacitance between the probe tip and the wafer surface. The potential of the probe tip is fixed during the scanning operation, so changes in the dV/dt term arise due to changes in the potential across the measured surface. Changes in the dC/dt term result from changes in the distance between the probe tip and the wafer surface, which most often result from changes in the height of the wafer surface. In most wafer surface scanning applications, the signal from capacitance changes is minimized by controlling the height of the probe tip above the wafer surface, minimizing wafer surface height variations, and/or minimizing the average voltage between the probe tip and the wafer surface through the application of a DC bias voltage. As a result, the capacitance signal is negligible and can be disregarded.
An important characteristic of the non-vibrating contact potential difference sensor is that it produces data that is differential; which means that it generates data that represents differences, or changes, in surface potential or work function across the measured surface. The output of the sensor represents changes in surface potential in the direction of travel of the sensor probe tip relative to the surface. The sensor output does not include any data on the change in surface potential in the direction perpendicular, or orthogonal, to the direction of travel of the sensor probe tip. Also, the sensor output does not provide data on the absolute contact potential difference between the probe tip and measured surface at any point. The sensor output only contains information on changes in surface potential.
The non-vibrating contact potential difference sensor relies on relative motion between the probe tip and measured surface to generate a signal. The act of moving the sensor probe tip parallel to the wafer surface to generate a signal is called scanning. There are numerous options for generating scanning motion between the probe tip and a wafer surface. For example, the wafer can be held fixed and the probe tip can be moved back-and-forth above the wafer surface to generate linear “tracks” of data, where a track is a continuous series of sequential data samples. Multiple linear tracks can be assembled into an image of the scanned surface. Alternatively, the probe can be held fixed and the wafer moved back-and-forth beneath the sensor probe tip. This type of scanning, where either the sensor or wafer is moved back-and-forth to produce a series of parallel linear scans, is often called raster scanning. Another option for generating the scanning motion is to rotate the wafer beneath the sensor probe tip, and move the sensor or wafer along a radius of the wafer to acquire a series of concentric circular tracks at different radii from the wafer center. These concentric tracks can then be assembled into an image of the scanned surface. This type of scanning operation is often called radial scanning because the probe tip is moved along a radius of the wafer.
With radial scanning, the spinning motion of the wafer provides relative motion between the probe tip and measured surface without the high accelerations and decelerations required by a raster scanning operation. Raster scanning requires accelerating the probe or wafer to the required scanning speed, acquiring a single track of data, and then decelerating and re-accelerating the probe or wafer in the opposite direction. With radial scanning, the wafer can be spun at a fixed or slowly varying speed, and the sensor can be moved small distances with low accelerations from one radial track to the next. As a result, the wafer surface can be scanned in a much shorter period of time with much less vibration and lower power consumption than with raster scanning.
The differential nature of the non-vibrating contact potential difference sensor signal means that a signal is generated only when the probe moves across a portion of the wafer surface where the surface potential changes from one location to another. If the sensor moves from an area with one surface potential value to an area with another surface potential value, a signal is generated only at the transition (edge) between the two regions. The differential sensor signal is proportional to the change in surface potential along the direction of motion of the probe. However, this differential signal can be converted to a new signal which is a linear function of relative surface potential by integrating the sensor signal. Integration is accomplished by computing the cumulative sum of consecutive samples. The integrated signal provides information on relative surface potentials in the direction of motion of the probe, but does not provide any information about surface variations perpendicular to the direction of motion, nor does it provide a measure of the absolute value of the contact potential difference. As a result of the scanning motion, any orthogonal variation is undetectable in the absence of additional measurements for determining that variation. In the case of raster-scanning, data on orthogonal variations can be acquired by performing the scanning operation twice: once in each of two perpendicular directions. However, this operation requires two scans of the surface, which doubles the scanning time. In the case of radial scanning, the scanning mechanics do not easily lend themselves to scanning each point on the wafer surface in two orthogonal directions. As a result, the circular motion of the probe relative to the wafer surface is not effective in detecting surface non-uniformities that vary radially. These types of radial variations in contact potential difference can arise from a variety of wafer processing steps. For example, dielectric charging caused by single wafer cleans or plasma processing operations can create a radial charge pattern that cannot be detected by the non-vibrating contact potential difference sensor using the radial scanning method.
As mentioned above, the non-vibrating contact potential difference sensor produces differential data that can be integrated to produce data representative of relative contact potential difference values across the surface. It is also possible to calibrate integrated non-vibrating contact potential difference data using vibrating contact potential difference measurements. Vibrating contact potential difference sensors are often called Kelvin probes, or Kelvin-Zisman probes. This type of sensor produces measurements of the absolute contact potential difference, in volts, between the probe tip and a particular point on the measured surface. Vibrating contact potential difference measurements, however, are very slow compared to non-vibrating contact potential difference measurements, and this technique is not suited to full-wafer imaging at production speeds. Integrated non-vibrating contact potential difference measurements can be transformed to provide absolute contact potential difference values by calculating a linear transformation between Kelvin probe measurements at multiple points and the integrated non-vibrating contact potential difference values at the same points on the measured surface. The best-fit linear transformation can be calculated using a technique such as least-squares line fitting. Once the best-fit linear transformation is calculated, it is applied to all points in the integrated non-vibrating contact potential difference image. This technique provides approximations of the absolute contact potential difference values for all scanned points, and is much faster than measuring the entire wafer surface using a vibrating contact potential difference sensor. The integrated non-vibrating data, however, still does not include any information on variations in surface potential perpendicular to the direction of motion of the probe tip. As a result, the integrated and transformed data will not include information on variations in surface potential that are perpendicular to the direction of motion of the scanning probe, and the resulting data will be incorrect if there are significant surface potential changes in this direction. This type of orthogonal variation in surface potential is common for radially scanned wafers because, as noted above, significant radial variations in surface potential can result from common semiconductor manufacturing processes. If significant radial variations exist, the correlation coefficient between the vibrating Kelvin probe measurements and the integrated radially-scanned non-vibrating contact potential difference data will be low, because the integrated non-vibrating contact potential difference image will not include this significant radial variation in surface potential.